Automated self seeding of batch crystallizations via plug flow seed generation

In this study, seed slurry from a single addition anti-solvent plug flow crystallization of benzoic acid was used to seed the equivalent batch cooling crystallization. The experimental conditions were carried out to simulate automated self-seeding. This involves withdrawal of solution from a batch crystallizer, which is then mixed with anti-solvent within a plug flow crystallizer, in order to generate a seed slurry which is fed directly back to the batch crystallizer. This seeding strategy allowed the final CSD of the batch crystallization to be controlled by variation of the crystal size from the plug flow seeding device at a constant seed loading. The ability to use unequal feed/anti-solvent inlet flowrates (in the Roughton vortex mixer) proved effective in controlling the batch CSD at 2% seed loading and constant feed composition.     

Monitoring milk fat fractionation: Filtration properties and crystallization kinetics

The effect of fractionation temperature, residence time, and agitation rate on the chemical composition of the stearin and olein milk fat fractions was studied. During fractionation, filtration properties of the crystal suspension were monitored; crystallization kinetics was determined by ¹H NMR. Higher fractionation temperatures result in a lower stearin yield, more oil entrapment, and a lower final solid fat content of the crystal suspension. On the other hand, the chemical composition of the resulting fractions is not influenced. Longer residence times lead to longer filtration times and lower oil entrapment, whereas the yield is not affected. Longer residence times induced lower growth rates, but chemical composition is not influenced. Agitation rates varying from 10 to 15 rpm have no influence on the chemical composition of stearin and olein milk fat fractions. Higher agitation rates decrease the filtration quality and increase stearin yield, causing a softer stearin. In designing and monitoring milk fat fractionation, filtration experiments and the assessment of crystallization kinetics are valuable techniques, but compositional chemical analysis is not favorable.     

Vergleich von Milchfetten aus der Kristallisationsfraktionierung und einem kontinuierlichen Fraktionierungsverfahren mittels überkritischem Kohlendioxid

Comparison of Milk Fat Fractions Obtained by Crystallization with those Obtained in a Continuous Fractionation Process Using Supercritical Carbon Dioxide Several milk fat fractions obtained by crystallization from molten fat as well as milk fat fractions obtained in a new continuous fractionation process using supercritical carbon dioxide have been investigated by GC, HPLC und DSC. The influence of its fatty acid and triglyceride compositions on the crystallization and melting behaviour is discussed. The milk fat fractions obtained by crystallisation tend to crystallize in two distinct main triglyceride group. A separation even into three fractions is shown in the melting thermograms. The milk fat fractions obtained by the fractionation using supercritical carbon dioxide have much less tendency to separate into triglyceride groups. Obviously, this continuous fractionation process using supercritical carbon dioxide can be improved. Therefore, it seems to be possible to obtain milk fat fractions with different triglyceride compositions and better physical properties.     

连续结晶过程非稳态特性的研究

建立了连续溶液结晶过程非稳态系统模拟软件,并应用该软件对过程内在不稳定性进行了分析.在10L及200L复杂结晶器内进行动态行为实验研究,观察晶粒粒度分布(CSD)的动态规律及系统参数对稳定性的影响趋向,实验与模拟分析的结果一致.     

木糖结晶动力学模型

<正>木糖(D-xylose)是一种新型的甜味剂.其生产原料为农业植物纤维废料,如玉米芯、甘蔗渣、棉子壳、甘蔗髓、稻壳等.中国是农业大国,木糖的原料极其丰富,生产木糖是一个具有较高附加值的二次资源利用项目.国内已有数家企业投入生产,但许多技术问题尚未解决,制约了经济效益,其中结晶工序也是瓶颈之一.本文作为木糖生产新工艺的重要环节,研究了木糖连续冷却结晶过程,并根据结晶动力学的粒数衡算理论(Population Balance)及其连续稳态的动力学测定方法,探索了木糖的结晶动力学模型.国内外迄今尚无这方面的报道.1 粒数衡算理论     

A systematic framework for design of process monitoring and control (PAT) systems for crystallization processes

A generic computer-aided framework for systematic design of a process monitoring and control system for crystallization processes has been developed to study various aspects of crystallization operations. The systematic design framework contains a generic crystallizer modelling toolbox, a tool for generation of the supersaturation set-point for supersaturation control, as well as a tool for design of a process monitoring and control system (also called Process Analytical Technology (PAT) system). This systematic design allows one to generate the necessary problem-chemical system specific model, the necessary supersaturation set-point as well as a PAT system design including implementation of monitoring tools and control strategies in order to produce the desired target product properties notably crystal size distribution (CSD) and shape for a wide range of crystallization processes. Application of the framework is highlighted through a case study involving the design of a monitoring and control system for a potassium dihydrogen phosphate (KDP) crystallization process, where also the one-dimensional CSD and two-dimensional CSD modelling features are highlighted.     

Parameters influencing cocrystallization and polymorphism in milk fat

Dry fractionation of milk fat is a common technique used to produce fat fractions with physical properties that are suitable for a variety of food and pharmaceutical products. During milk fat fractionation, the partial crystallization of triacylglycerols from the melt is the most important step. The efficiency of the separation of the crystals from the suspension is also important, but the crystallization itself influences the chemical composition and thereby determines the properties of the crystal fraction. At low supercooling, the crystallization kinetics are slow, and thus process time is increased. With increased kinetics due to a strong supersaturation, the chemical composition of the crystals is changed compared to crystals formed under slow kinetic conditions. This study shows to what extent controlled temperature and supercooling during milk fat crystallization influence crystal amount and the physical properties of the resulting fractions. Differences of the thermal characteristics of the crystal suspensions are directly detected by differential scanning calorimetry and nuclear magnetic resonance. At slow crystallization kinetics, the melting temperature range of the crystals in the suspensions is broader, and the resolution of the melting peak is higher. It is shown that compound crystals are formed when supercooling is performed, even if the supercooling takes place only for a short period of time. Controlled temperature conditions during crystallization govern larger differences in the fatty acid and triacylglycerol composition of the liquid and of the crystalline phases, compared to fractions crystallized under supercooling conditions, which contain a high amount of compound crystals.     

Milk fat fractionation by solid-layer melt crystallization

The layer crystallization process has the potential to produce the same milk fat fractions as can be obtained by the suspension crystallization process. That is, milk fat fractions with solid fat content melting profiles similar to those obtained by suspension fractionation can be produced with this technique. The fatty acid profiles as well as the melting enthalpies of the different fractions confirm the separation of milk fat by the layer technique. Furthermore, there is potential to improve the results of separation presented in the first part of this paper. The two sources of improvement, temperature control of the process and controlled nucleation, lead to (i) a smooth crystalline layer with a low amount of entrapped mother liquor, contrary to the layers composed of agglomerated needles, and (ii) a good quality of attachment of the crystalline layer to the cooled surface. Moreover, the product quality can be increased using sweating as a postcrystallization step. “Sweating by warm gas” seems to have a better outlook concerning handling and controlling the process than “sweating by warm tube” because sloughing of the crystal layers can be avoided. Further investigations of the mass ratio of sweating fraction and amount of product as well as the aspect of energy consumption will determine the technical feasibility of solid-layer crystallization for fractionation of milk fat.     

Releasing Fat in Whole Milk Powder during Fluidized Bed Drying

Spray-dried whole milk powders have been produced to a high bulk-to-surface-free-fat ratio due to the susceptibility to oxidation of surface free fat. Processing spray-dried whole milk powders in a fluidized-bed crystallizer by hot and high humidity air can cause the release of bulk fat and increase both inner free and surface free fat. This research investigates the fat release and its migration to particle surfaces by two different mechanisms: a) fat melting due to high temperature processing in a fluidized -bed crystallizer so that liquid fat can diffuse outwards to the particle surfaces; b) fat release from the matrix due to lactose crystallization, causing component segregation and releasing molecular bonds.

More free fat is preferable in some dairy-based processing, such as chocolate making, where cocoa butter could be substituted by milk fat to adjust the viscosity of chocolate paste and formulate good chocolate taste. This research shows the possibility of releasing more than 90% of the bulk fat to increase free fat (inner and surface) in 15 minutes processing time. Different temperatures and humidities (45°C and 78% RH) have been found to provide suitable conditions to release the bulk fat by crystallizing lactose in higher amount, as a form of inner free fat (preferentially) which is ready for further use, such as in chocolate industries. Meanwhile the released fat, as inner free fat, has less contact with oxygen than surface free fat and maintains the powder quality better. The changes in particle structure and fat release processes have also been studied during fluidized-bed drying of spray-dried whole milk powders that could improve the control and design of the drying process for milk powders.     

Iterative parameter estimation for extraction of crystallization kinetics of potassium chloride from batch experiments

Extraction of crystallization kinetics, the fundamental information which governs the performance of a crystallization process, is characterized by experimental difficulties in MSMPR and mathematical difficulties in batch crystallization. Here, a rigorous approach is taken to estimate kinetic parameters from a batch crystallizer. The two step LaxWendorff technique is adapted for the solution of the population balance in a batch crystallizer and an iterative self-correcting least squares algorithm is implemented for the estimation of the kinetic parameters. The need for multi-response estimation as opposed to single-response from terminal CSD is demonstrated. The kinetics extracted are average ones, representing primary and secondary nucleation kinetics. It is found that the kinetic parameters estimated by multi-response technique have a superior predictive capability as opposed to those obtained using the terminal CSD only. An advantage of the proposed algorithm is that the measurement of course of supersaturation, which is difficult to perform, is not required.     

Über die physikalischen und chemischen Eigenschaften der Milchfettfraktionen

Physical and Chemical Characteristics of Milk Fat Fraction Milk fat is a mixture of many different triglycerides, and its crystallization characteristics offer good possibilities for fat fractionation. In principle, this can be done in several differnt ways. In our studies milk fat was mostly divided into two or more fractions through crystallization and separation as the temperature was gradually lowered. Fractionation was carried out without solvents or additives. The solidification and melting characteristics of the fractions obtained, as well as their fatty acid and triglyceride composition, differed considerably from each other and from the corresponding characteristics of the original milk fat. By combining different milk fat fractions, the melting characteristics of fat can be led in the desired direction, making it possible to created fat mixtures suitable for a variety of special applications. Is 1987 we have carried out soem experiments for the fractionation of milk fat using the supercritical carbon dioxide extraction. This extraction may offer new possibilities for additional uses of milk fat.     

Product quality improvement of batch crystallizers by a batch-to-batch optimization and nonlinear control approach

Batch crystallization is one of the widely used processes for separation and purification in many chemical industries. Dynamic optimization of such a process has recently shown the improvement of final product quality in term of a crystal size distribution (CSD) by determining an optimal operating policy. However, under the presence of unknown or uncertain model parameters, the desired product quality may not be achieved when the calculated optimal control profile is implemented. In this study, a batch-to-batch optimization strategy is proposed for the estimation of uncertain kinetic parameters in the batch crystallization process, choosing the seeded batch crystallizer of potassium sulfate as a case study. The information of the CSD obtained at the end of batch run is employed in such an optimization-based estimation. The updated kinetic parameters are used to modify an optimal operating temperature policy of a crystallizer for a subsequent operation. This optimal temperature policy is then employed as new reference for a temperature controller which is based on a generic model control algorithm to control the crystallizer in a new batch run.     

Crystallization of alpha-lactose monohydrate in a drop-based microfluidic crystallizer

A microfluidic process for producing crystals of controlled size by confining the crystallization within drops is demonstrated. The process consists of a drop producing stage which segments the mother liquor into monodisperse drops followed by crystallization in a temperature controlled tubular crystallizer. The process is implemented to produce lactose crystals with significantly narrower crystal size distribution (CSD) as compared with crystals produced in a stirred bulk crystallizer. By controlling the drop size and initial supersaturation the mean crystal size can be controlled. The distribution of the number of crystals per drop and the CSD are measured as a function of supersaturation at temperatures between 20 and for 150 and drops. In the case where drops contain only one crystal, a very narrow CSD is obtained with a coefficient of variation of crystal size as low as 7%. The crystallization of lactose in a microfluidic tubular crystallizer is modeled by treating nucleation in drops as a Poisson process with a nucleation rate based on classical nucleation theory. Experimental results are in good agreement with predictions from a Poisson process model over the range of temperatures, supersaturations and drop sizes tested.     

Extraktion von Milchfett mit überkritischem Kohlendioxid

Extraction of Milk Fat with Supercritical Carbon Dioxide In parallel with fractionation based on crystallization, studies have been made in recent years on milk fat fractionation with supercritical carbon dioxide. This means that the fat is chiefly extracted on the basis of molecular size. The extraction experiments were carried out using a pilot plant extraction apparatus. The milk fat was extracted in 4 or 2 stages. The melting properties of extracts are explained by the compositions of fatty acids and triacylglycerols. The percentages of short chain fatty acids C4–C10 becomes smaller as the extractio proceeds. The percentage of palmitic acid remains virtually unchanged at all stages of the extraction. The percentages of C 18 fatty acids, stearic and oleic acids increase as the extraction proceeds. Cholesterol and aroma compounds of milk fat were clearly enriched in the first extract. Supercritical extraction opens up some new potential for milk fat technology and for the food industry using milk fat. At this stage it wold seem especially interesting to study the possible uses of extracts rich in aroma.     

基于实验设计的L-谷氨酸连续结晶过程优化

对于采用连续振荡挡板式结晶器进行β型L-谷氨酸的冷却结晶,提出一种基于实验设计的晶体产品尺寸分布预测模型和过程操作条件优化方法。将连续管式结晶器的振荡频率、2个区段的降温速率作为操作变量,设计一个包含3因素3水平的响应面实验方案。通过检测不同实验条件下的晶体产品弦长分布,构建一个基于双层基函数的晶体产品尺寸分布预测模型。然后引入一个关于目标晶体尺寸分布和产品收率的目标函数,并且利用上述预测模型,建立一个优化过程操作条件的方法。通过对β型L-谷氨酸冷却结晶过程的仿真和实验,验证了该方法的有效性和优点。     

Dynamic study and control of crystal size distribution (CSD) in a KCL crystallizer

Previous works on crystal size distribution (CSD) control have indicated that a successful control scheme requires on-line measurement of nuclei density. Such measurements can only be performed using an extremely expensive particle size analyzer. Besides the cost, the accuracy of such analyzers is restricted to low concentration slurries. In the present work an attempt is made to replace nuclei density by an alternative process variable which can be measured easily and inexpensively. Theoretical analysis of a KCl crystallizer shows that slurry density in the fines loop is an excellent substitute for nuclei density. Preliminary tests show that a turbidimeter can be used for measurement of slurry density in the fines loop. Proportional direct feedback control based on measurement of slurry concentration in the fines loop and manipulation of fines removal rate is shown to stabilize an inherently unstable crystallizer. The ability of the control scheme to suppress outside disturbances is also verified.     

Micromixing limits in an MSMPR crystallizer

The effect of micromixing limits on a process of crystallization in an MSMPR crystallizer is studied with respect to power law growth and nucleation kinetics. Three limiting cases corresponding to maximum mixedness and complete segregation in an MSMPR crystallizer and plug flow configuration were analyzed for processes in which supersaturation is generated by conventional techniques. The sensitivity of these three limiting cases to the supersaturation generation term in each mode of operation was investigated using several numerical examples. The study demonstrates the effects of mixing on the overall crystallizer performance and, in particular, the enormous micromixing influence at high supersaturation generation rates. The difference in the product CSD arise from the variations of supersaturation profiles experienced by the elementary volumes throughout their sojourn. Characterization of mixing in a real crystallizer at some intermediate levels is emphasized with the aid of relevant industrial examples.     

Kinetics of butterfat crystallization

Fractionation of butterfat by melt crystallization is a commercial process in many countries for making butter fractions with varying melting, textural and flavor properties for use as food ingredients. However, the crystallization phenomena in this complex system are poorly understood and difficult to optimize and control. In this study, the crystallization kinetics of anhydrous butterfat were determined by cooling a melted sample to the final crystallization temperature in either a lab-scale (2 L) batch crystallizer or a pilot-scale (20 L) crystallization vessel. The butterfat was cooled sequentially from an initial temperature of 60°C to final temperatures of 30, 20 and 15°C at a constant cooling rate. Crystals formed at each temperature were separated by vacuum filtration, with the liquid cooled to the next crystallization temperature. Nucleation rates were determined by counting the number of crystals in a given volume of suspension during the course of crystallization. Crystal growth rates were obtained from image analysis of optical photomicrographs. Changes in viscosity, turbidity and mass of crystals also were determined. Effects of impeller velocity (75, 100 or 125 rpm) on the crystallization kinetics were determined. Nucleation and mass deposition rates increased while crystallization lag times decreased with increasing agitator velocities. Growth rates increased with agitator rpm at 20 and 15°C, but decreased with agitator rpm at 30°C, indicating different growth mechanisms. At 20 and 30°C, aggregation was the primary mechanism of crystal growth, whereas little aggregation was observed at 15°C. Crystallization at the larger scale, 20 L, showed only minor differences.     

Crystallization and fractionation of milk fat

Recent progress in understanding milk fat crystallization and fractionation is reviewed. Extent of fat solidification in butter can be altered by variations in thermal treatment of cream prior to churning. Because of its compositional complexity, milk fat rarely exhibits polymorphism. As with mixtures of closely related triglycerides, milk fat forms solid solutions. A typical milk fat begins melting below −40 C, maximum melting occurs at 15–18 C, and the highest melting fraction appears 20–37 C as a shoulder on the main peak. Dispersion of fat in emulsions increases its tolerance to supercooling, thereby altering the properties and composition of the solid phase. Most studies of milk fat fractionation have used progressive fractional crystallization, either of the melt or of solutions. Both procedures result in fractions showing larger changes in mp than in composition. The high melting glyceride fraction, ca. 5% total fat, influences crystallization out of proportion to concentration. The Alfa-Laval system, using an aqueous suspension of partially crystalline fat, produces two fractions. Typical high melting fractions have softening points ca. 3C higher than the original fat. The softening point of typical low melting fractions is lowered 10 C. Refractionation is easier with the high melting fraction. Melting thermograms of these fractions show them as resembling fractions prepared from melted fat. One of eight papers presented at the Symposium “Milk Lipids,” AOCS Fall Meeting, Ottawa, Canada, September 1972.     

Tempering method for chocolate containing milk-fat fractions

Anhydrous milk fat (AMF) was fractionated by a two-stage dry fractionation process to produce three fractions—high-(HMF), middle-(MMF), and low-melting (LMF). The effect of replacing 12.2–40% by weight of cocoa butter with these fractions on the tempering profile of milk chocolate was studied. Degree of temper was evaluated by differential scanning calorimetry, and expressed as the ratio of enthalpies of melting for higher-stability polymorphs to those of lesser stability. The degree of temper was dependent on the crystallization time and temperature, and the type and quantity of milk-fat fraction in the formulation. Chocolates containing AMF or its fractions in concentrations of up to 20 wt% (total fat basis) were tempered after a conventional thermocycling tempering process (50°C/30 min, 27.7°C/4 min, 31°C/2 min) to obtain products with good contraction and mold release properties. For those milk chocolate formulations that did not temper by the conventional method and resulted in poor contraction and mold release, a new tempering protocol was developed. Lower crystallization temperatures and/or longer holding times were required at concentrations of AMF, MMF, or LMF above 20%. Chocolate containing HMF required slightly higher crystallization temperatures because of high viscosity. Chocolates containing up to 35% HMF and up to 40% of the total weight of fat in the chocolate of AMF, MMF, and LMF were successfully tempered by adjusting crystallization time and temperature.